Solution for making photoconductive layer and an electrophotographic manufacturing method of the layer in CRT

ABSTRACT

Disclosed is a photoconductive solution which has excellent charge characteristics with easy control of charge amount and is completely volatilized after baking. The solution contains 2 to 5% by weight of tetraphenyl ethylene as ultraviolet-sensitive material. 0.1 to 1 wt. % diphenylpicrylhydrazine or 0.1 wt. % or less of tetracyanoquino-dimethane(TCNQ) as ultraviolet-sensitive donor material is desirably added to the tetraphenyl ethylene of 2 to 5 wt. % for better accomplishment of the purpose, and the solution contains at least one of trinitrofluorenone, ethylanthraquinone and their mixture of 0.1 to 1 wt. % as an acceptor. The solution is formed by mixing the above ingredients together with 10 to 20 wt. % of polystyrene-oxazoline copolymer(PS-OX) as polymer binder, and 20 to 85 wt. % of toluene as solvent.

FIELD OF THE INVENTION

The present invention relates to a solution for making a photoconductivelayer and a method of electrophotographically manufacturing a viewingscreen for a cathode ray tube(CRT) using the solution, and moreparticularly to a photoconductive solution which has higher chargecharacteristics by a corona discharger with a similar photoconductivityto one in the prior art.

BACKGROUND OF THE INVENTION

Referring to FIG. 1, a color CRT 10 generally comprises an evacuatedglass envelope consisting of a panel 12, a funnel 13 sealed to the panel12 and a tubular neck 14 connected by the funnel 13, an electron gun 11centrally mounted within the neck 14 and a shadow mask 16 removablymounted to a sidewall of the panel 12. A three color phosphor screen isformed on the inner surface of a display window or faceplate 18 of thepanel 12.

The electron gun 11 generates three electron beams 19a or 19b, saidbeams being directed along convergent paths through the shadow mask 16to the screen 20 by means of several lenses of the gun and a highpositive voltage applied through an anode button 15 and being deflectedby a deflection yoke 17 so as to scan over the screen 20 throughapertures or slits 16a formed in the shadow mask 16.

In the color CRT 10, the phosphor screen 20, as shown in FIG. 2,comprises an array of three phosphor elements R, G and B of threedifferent emission colors arranged in a cyclic order of a predeterminedstructure of multiple-stripe or multiple-dot shape and a matrix oflight-absorptive material surrounding the phosphor elements R, G and B.

A thin film of aluminum 22 overlies the screen 20 in order to provide ameans for applying the uniform potential applied through the anodebutton 15 to the screen 20, increase the brightness of the phosphorscreen and prevent from degrading ions in the phosphor screen anddecreasing the potential of the phosphor screen. And also, a film ofresin such as lacquer(not shown) may be applied between the aluminumthin film 22 and the phosphor screen to enhance the flatness andreflectivity of the aluminum thin film 22

In a photolithographic wet process, which is well known as a prior artprocess for forming the phosphor screen, a slurry of a photosensitivebinder and phosphor particles is coated on the inner surface of thefaceplate. It does not meet the higher resolution demands and requires alot of complicated processing steps and a lot of manufacturingequipments, thereby necessitating a high cost in manufacturing thephosphor screen. And also, it discharges a large quantity of effluentsuch as waste water, phosphor elements, 6th chrome sensitizer, etc.,with the use of a large quantity of clean water.

To solve or alleviate the above problems, the improved process ofelectrophotographically manufacturing the screen utilizing dry-powderedphosphor particles is developed. U.S. Pat. No. 4,921,767, issued toDatta at al. on May 1, 1990, describes one method ofelectrophotographically manufacturing the phosphor screen assembly usingdry-powdered phosphor particles through the repetition of a series ofsteps represented in FIGS. 3A to 3E, as is briefly explained in thefollowing(FIG. 3D and FIG. 3E respectively show a developing step and afixing step described in our copending Korean patent application SerialNo. 95-10420 filed on Apr. 29, 1995 and assigned to the assignee of thepresent invention.)

Prior to the electrophotographic screening process, foreign substance isclearly removed from an inner surface of a panel by several conventionalmethods. Then, a conductive layer 132, as shown in FIG. 3A, is formed byconventionally coating the inner surface of the viewing faceplate 18with a suitable conductive solution comprising an electricallyconductive material which provides an electrode for an overlyingphotoconductive layer 134. The conductive layer 132 can be an inorganicconductive material such as tin oxide or indium oxide, or their mixtureor, preferably, a volatilizable organic conductive material consistingof a polyelectrolyte commercially known as polybrene(1,5-dimethyl-1,5-diaza-undecamethylene polymethobromide, hexadimethrinebromide), available from Aldrich Chemical Co., Milwaukee Wis., oranother quaternary ammonium salt. The polybrene is conventionallyapplied to the inner surface of the viewing faceplate 18 in an aqueoussolution containing about 10 percent by weight of propanol and about 10percent by weight of a water soluble, adhesion promoting polymer such aspoly(vinyl alcohol), polyacrylic acid, certain polyamide and the like,and the coated solution is dried to form the conductive layer 132 havinga thickness from about 1 to 2 microns and a surface resistivity of lessthan about 10⁸ ohms per square unit.

The photoconductive layer 134 is formed by coating the conductive layer132 with a photoconductive solution comprising a volatilizable organicpolymeric material, a suitable photoconductive dye and a solvent. Thepolymeric material is an organic polymer such as polyvinyl carbazole, oran organic monomer such as n-ethyl carbazole, n-vinyl carbazole ortetraphenylbutatriene dissolved in a polymeric binder such aspolymethyl-methacrylate or polypropylene carbonate. The suitable dyes,which are sensitive to light in the visible spectrum, preferably fromabout 400 to 700 nm, include crystal violet, chloridine blue, rhodamineEG and the like. This dye is typically present in the photoconductivecomposition in from about 0.1 to 0.4% by weight. The solvent for thephotoconductive composition is an organic such as chlorobenzene orcyclopentanone and the like which will produce as little crosscontamination as possible between the layers 132 and 134. Thephotoconductive solution is conventionally applied to the conductivelayer 132, as by spin coating, and dried to form a layer having athickness from about 2 to 6 microns.

FIG. 3B schematically illustrates a charging step, wherein thephotoconductive layer 134 overlying the conductive layer 132 ispositively charged in a dark environment by a conventional positivecorona discharger 136, which moves across -the layer 134 and charges itwithin the range of +200 to +700 volts.

FIG. 3C schematically shows an exposure step, wherein the shadow mask 16is inserted in the panel 12 and the charged photoconductor is exposedthrough a lens system 140 and the shadow mask 16, to the light from axenon flash lamp 138 disposed at one position within a conventionalthree-in-one lighthouse. Then, the positive charges of the exposed areasare discharged through the grounded conductive layer 132 and the chargesof the unexposed areas remain in the photoconductive layer 134, thusestablishing a latent charge image in a predetermined array structure.Three exposures are required for forming a light-absorptive matrix withthree different incident angles, respectively.

FIG. 3D diagrammatically illustrates the outline of a developing step,as described in the Korean patent application Serial No. 95-10420 citedabove. In FIG. 3D, after removing the shadow mask 16, suitably charged,dry-powdered particles such as particular color-emitting phosphorparticles or light-absorptive material particles are sprayed bycompressed air toward a photoconductive layer 134 through a venturi tube146 and a nozzle 144b from a hopper 148 and attracted to one of thecharged or unexposed areas and the discharged or exposed areas dependingupon the polarity of the charged particles due to electrical attractionor repulsion, thus one of the two areas is developed in a predeterminedarray pattern. Below the nozzle 144b, there is provided a dischargeelectrode 144a such as a corona discharger for charging dry-powderedparticles to be sprayed in the nozzle 144b. The light-absorptivematerial particles for directly developing the unexposed or positivelycharged areas are negatively charged and the phosphor particles arepositively charged for reversely developing the exposed or dischargedareas. The charging of the dry-powdered particles may be executed by atriboelectrical charging method using surface-treated carrier beads, asdisclosed in U.S. Pat. No. 4,921,767 cited above.

FIG. 3E schematically illustrates a fixing step using a vapor swellingmethod, as described in the Korean patent application serial No.95-10420 cited above.

In the fixing step, the surface of polymers contained in thephotoconductive layer 134 are dissolved by coming into contact withsolvent vapor such as acetone, methyl isobutyl ketone, etc., on thesurface of the developed photoconductive layer 134, said dissolvedpolymers fixing the dry-powdered particles deposited on the developedareas of the photoconductive layer 134.

The fixing step also may be executed by infrared radiation to fix thedeposited particles by melting or thermally bonding the polymercomponents of the particles 21 and the photoconductive layer 134 to thephotoconductive layer 134, as disclosed in U.S. Pat. No. 4,921,767 citedabove.

The steps of charging, exposing, developing and fixing are repeated forthe black matrix particles and the three different phosphor particles.The faceplate panel 12 is baked in air at a temperature of 425 degreescentigrade, for about 30 minutes to drive off the volatilizableconstituents of screen including the conductive layer 132, thephotoconductive layer 134, the solvents present in both the screenstructure materials and in the filming lacquer, thereby forming anscreen array of light-absorptive material 21 and three phosphor elementsR, G and B in FIG. 2.

The aforementioned process, as disclosed in U.S. Pat. No. 4,921,767cited above, has one problem that it requires dark environment duringperforming all the steps since the photoconductive layer is sensitive tothe visual light.

Korean patent application serial No. 95-10420, cited above, and U.S.Pat. No. 5,413,885 disclose a method of electrophotographicallymanufacturing the CRT screen under visible lights or low intensityyellow lights of 577-597 nm using a novel photoconductive layer to solvethe aforementioned problem. The photoconductive layer is formed byapplying a photoconductive solution containing bis dimethyl phenyldiphenyl butatriene as a donor of ultraviolet-sensitive material, andone of trinitro fluorenone (TNF), ethylanthraquinone (EAQ) and theirmixture as an acceptor with polystyrene as polymer binder.

The photoconductive solution, which, as described in FIG. 3A, containsthe organic polymer or an organic monomer such as n-ethyl carbazole,n-vinyl carbazole or tetraphenylbutatriene dissolved in a polymericbinder such as polymethyl-methacrylate or polypropylene carbonate, andthe suitable dyes sensitive to light, or which contains bis dimethylphenyl diphenyl butatriene and one of trinitro fluorenone(TNF),ethylanthraquinone(EAQ) and their mixture with polystyrene, is appliedto the conductive layer 132, thereby the photoconductive layer 134 beingformed.

However, since said photoconductive layer 134 has low chargecharacteristics and the applied potential is limited in order to preventthe damage of the photoconductive layer 134, there are some problemsthat it takes much time to charge the photoconductive layer 134 with thecorona discharger 144a in FIG. 3B and the whole surface of thephotoconductive layer 134 is not charged uniformly. Also, furtherproblem is that said bis-1,4-dimethyl phenyl(-1,4-diphenyl(butatriene))is not volatilized perfectly after burning in the frit step of bulb and8 or 10wt. % thereof remains on the screen structure of the panel.

In order to remove the aforementioned problems, it is an object of thepresent invention to provide a photoconductive solution which hasexcellent charge characteristics with easy control of charge amount andis completely volatilized after baking.

SUMMARY OF THE INVENTION

To accomplish the aforementioned purpose, the present invention providesa solution for forming a photoconductive layer forelectrophotographically manufacturing a luminescent screen on aninterior surface of a faceplate panel for a CRT comprising the steps ofcoating said surface of the panel with a volatilizable conductive layerand an overlying volatilizable photoconductive layer, establishing asubstantially uniform electrostatic charge over the whole area of theinner surface of said photoconductive layer, exposing selected areas ofsaid photoconductive layer to discharge the charge from the selectedareas, developing one of the charged, unexposed areas and thedischarged, exposed areas depending upon the polarity of the chargedparticles with one of charged phosphor particles and light-absorptivematerial particles, said solution containing 2 to 5% by weight oftetraphenyl ethylene as ultraviolet-sensitive material.

In said solution, 0.1 to 1 wt. % diphenylpicryl-hydrazine(DPPH) isdesirably added to said tetraphenyl ethylene of 2 to 5 wt. % for betteraccomplishment of the purpose, and the solution may contain at least oneof trinitrofluorenone(TNF), ethyl-anthraquinone(EAQ) and their mixtureby 0.1 to 1 wt. % respectively as an acceptor.

Said solution is formed by mixing the above ingredients together with 10to 20 wt. % of polystyrene-oxazoline copolymer(PS-OX) as polymer binder,and 20 to 85 wt. % of toluene as solvent.

The present invention further provides a solution for forming aphotoconductive layer for, and a method of, electrophoto-graphicallymanufacturing a luminescent screen on an interior surface of a faceplatepanel for a CRT comprising the steps of coating said surface of thepanel with a volatilizable conductive layer and an overlyingvolatilizable photoconductive layer, establishing a substantiallyuniform electrostatic charge over the whole area of the inner surface ofsaid photoconductive layer, exposing selected areas of saidphotoconductive layer to discharge the charge from the selected areas,developing one of the charged, unexposed areas and the discharged,exposed areas depending upon the polarity of the charged particles withone of charged phosphor particles and light-absorptive materialparticles, said solution comprising 2 to 5% by weight of tetraphenylethylene, diphenylpicryl-hydrazine(DPPH) of 0.1 to 1 wt. % as a donor,at least one of 0.1 to 1 wt. % of trinitro-fluorenone(TNF), 0.1 to 1 wt.% of ethylanthraquinone(EAQ) and 0.1 to 1 wt. % of their mixture as anacceptor, 10 to 20 wt. % of polystyrene-oxazoline copolymer(PS-OX) aspolymer binder, and 20 to 85 wt. % of toluene as solvent.

In the foregoing solution and method, said overlying volatilizablephotoconductive layer can be formed by applying a solution comprising 2to 5% by weight of tetraphenyl ethylene and 0.1 wt. % or less oftetracyanoquino-dimethane(TCNQ) as ultraviolet-sensitive donor material.In this case, as an acceptor, at least one of 0.1 to 1 wt. % oftrinitrofluorenone(TNF), 0.1 to 1 wt. % of ethylanthraquinone(EAQ) and0.1 to 1 wt. % of their mixture and 10 to 20 wt. % ofpolystyrene-oxazoline copolymer(PS-OX) as polymer binder are comprised,and 20 to 85 wt. % of toluene is used as solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view partially in axial section of a color cathode-raytube.

FIG. 2 is an enlarged section of a screen assembly of the tube shown inFIG. 1.

FIGS. 3A through 3E show various steps in electrophotographicallymanufacturing the screen assembly of the tube by viewing a portion of afaceplate having a conductive layer and an overlying photoconductivelayer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As described above relating to FIG. 3A, the interior surface of a panel18 is coated with a volatilizable conductive layer 132 and an overlyingvolatilizable photoconductive layer 134. Said photoconductive layer 134is formed by applying a photoconductive solution to the conductive layer132.

According to one embodiment of the present invention, said solution isprepared by dissolving 2 to 5% by weight of tetraphenyl ethylene asultraviolet-sensitive donor material, at least one of. 0.1 to 1 wt. % oftrinitrofluorenone (TNF), 0.1 to 1 wt. % of ethylanthraquinone (EAQ) and0.1 to 1 wt. % of their mixture as an acceptor, and 10 to 20 wt. % ofpolystyrene-oxazoline copolymer(PS-OX), in 20 to 85 wt. %(balance) oftoluene as solvent.

According to another embodiment of present invention, a photoconductivesolution comprises 2 to 5% by weight of at least one of tetraphenylethylene and 0.1 wt. % or less of tetracyanoquinodimethane(TCNQ) as adonor, at least one of 0.1 to 1 wt. % of trinitrofluorenone(TNF), 0.1 to1 wt. % of ethylanthraquinone(EAQ) and 0.1 to 1 wt. % of their mixtureas an acceptor, 10 to 20 wt. % of polystyrene-oxazolinecopolymer(PS-OX), and 20 to 85 wt. % of toluene as solvent. Suchsolution is applied to the conductive layer 132 by the conventionalmethod, thereby forming the photoconductive layer 134.

Said tetraphenyl ethylene acts as a donor and saidtrinitrofluorenone(TNF) or ethylanthraquinone (EAQ) acts as an acceptorwhen exposed to ultraviolet rays. Such solution is applied to theconductive layer 132 by the conventional method, thereby forming thephotoconductive layer 134.

Thus, the photoconductive layer 134 formed by applying the solution isshown to have excellent charge or electric characteristics in thecharging step of FIG. 3B and is almost volatilized after performing aseries of the exposing step(FIG. 3C), the developing step(FIG. 3D), thefixing step(FIG. 3E) and the baking step(not shown). That is, although,in the case of the prior bis dimethyl phenyl diphenyl butatriene, thecharge voltage or breakdown voltage is 140 volts per 1 micron of itsthickness and the residual potential 20 volts, they are shown to bearound 160 volts per 1 micron of its thickness and around 40 volts inthe case of the tetraphenyl ethylene according to the present invention.Particularly, when diphenylpicrylhydrazine(DPPH) of 0.1 to 1 wt. % or0.1 wt. % or less of tetracyanoquino-dimethane(TCNQ) asultraviolet-sensitive donor material is added to the tetraphenylethylene, the photoconductive layer 134 is show to have far moreexcellent charge or electric characteristics.

Also, the tetraphenyl ethylene according to the present invention isperfectly burned and volatilized without any residual substance at 400degrees centigrade.

In the foregoing embodiments, in the case of containing below 2 wt. % oftetraphenyl ethylene, the photoconductive layer does not act as theultraviolet-sensitive layer, and in the case of over 5 wt. % of thetetraphenyl ethylene, foreign substance undesirably comes into existenceand is coagulated or bubble is generated on the photoconductive layer.

The aforementioned solutions according to the aforementioned embodimentsof the present invention are used in electrophotographicallymanufacturing a luminescent screen on an interior surface of a faceplatepanel for a CRT as described in the following.

In FIG. 3A, the inner surface of a panel 18 is coated with avolatilizable conductive layer 132 as described in the forgoing priorart and then with an overlying volatilizable photoconductive layer 134using any one of the forgoing solutions of the present invention. Thephotoconductive layer 134 is uniformly and quickly charged with positiveelectrostatic charge over the whole area of the inner surface thereof bythe corona discharger 144a and then, said photoconductive layer isexposed in selected areas thereof to discharge the charge from theselected areas, developing one of the charged, unexposed areas and thedischarged. The exposed areas are developed with charged phosphorparticles and said developed phosphor particles are fixed on thephotoconductive layer 134, such steps being performed under the visuallight.

The steps of charging, exposing, developing and fixing are repeated forthe black matrix particles and the three different phosphor particles.After the screen is formed using said photoconductive solution by themethod as described in relation to FIGS. 3A to 3E, a spray film oflacquer and an overlying aluminum thin film are formed on the screen asis known in the art. The screen is baked at a high temperature, as isknown in the art and then the volatilizable constituents of the screenincluding the conductive layer 132, the photoconductive layer 134, etc.,are completely driven off, thus the screen being formed with thelight-absorptive black matrix 21 and an array of the three differentphosphor elements R, G and B and without any other foreign substance asillustrated in FIG. 2.

The aforementioned solutions of the present invention facilitatecontrolling of charge in the charging step of FIG. 3B, and develop thecharge characteristics of the photoconductive layer with maintaining thecharge in the photoconductive layer for a long time. Also, saidsolutions can be completely removed from the screen, thus improving thequality of the CRT's screen.

It should be clear to one skilled in the art that the present solutionscan be altered and applied without any limitation to the aforementionedembodiments of the present invention and within the scope of the presentinvention's spirit. For example, the present solution can be used forelectrophotographically manufacturing the screen by the method asdescribed in U.S. Pat. No. 4,921,767, cited above.

What is claimed is:
 1. A solution for forming a photoconductive layerfor electrophotographically manufacturing a luminescent screen on aninterior surface of a faceplate panel for a CRT created by coating theinterior surface of the faceplate panel with a volatilizable conductivelayer and an overlying volatilizable photoconductive layer, establishinga substantially uniform electrostatic charge over the whole area of theinner surface of the photoconductive layer, exposing selected areas ofthe of the photoconductive layer to discharge the charge from theselected areas, developing one of the charged, unexposed areas and thedischarged, exposed areas with one of charged phosphor particles andlight-absorptive material particles, depending upon the polarity of thecharged particles, the solution comprising:2-5% by weight of tetraphenylethylene as an ultraviolet-sensitive donor material; and 0.1-1% byweight of diphenylpicrylhydrazine.
 2. A solution according to claim 1,further comprising 0.1-1% by weight of an acceptor selected from thegroup consisting of trinitrofluorenone (TNF), ethylanthraquinone (EAQ),and mixtures thereof.
 3. A solution according to claim 2, furthercomprising 10-20% by weight of polystyrene-oxazoline copolymer (PS-OX)as a polymer binder and 20-85% by weight of toluene as a solvent.
 4. Asolution for forming a photoconductive layer for electrophotographicallymanufacturing a luminescent screen on an interior surface of a faceplatepanel for a CRT created by coating the interior surface of the faceplatepanel with a volatilizable conductive layer and an overlyingvolatilizable photoconductive layer, establishing a substantiallyuniform electrostatic charge over the whole area of an inner surface ofthe photoconductive layer, exposing selected areas of thephotoconductive layer to discharge the charge from the selected areas,developing one of the charged, unexposed areas and the discharged,exposed areas with one of charged phosphor particles andlight-absorptive material particles, depending upon the polarity of thecharged particles, the solution comprising:2-5% by weight of tetraphenylethylene and 0-0.1% by weight of tetracyanoquino-dimethane (TCNQ) as anultraviolet-sensitive donor material; 0.1-1.0% by weight of an acceptorselected from the group consisting of trinitrofluorenone (TNF),ethylanthraquinone (EAQ), and mixtures thereof; 10-20% by weight ofpolystyrene-oxazoline copolymer (PS-OX)as a polymer binder; and 20-85%by weight of toluene as a solvent.
 5. A method ofelectrophotographically manufacturing a luminescent screen on aninterior surface of a faceplate panel for a CRT, the methodcomprising:coating the interior surface of the faceplate panel with avolatilizable conductive layer and an overlying volatilizablephotoconductive layer, the overlying volatilizable photoconductive layerbeing formed by applying and drying a solution having 2-5% by weight oftetraphenyl ethylene and 0-0.1% by weight of diphenylpicryl-hydrazine asa donor material, 0.1-1.0% by weight of an acceptor selected from thegroup consisting of trinitrofluorenone (TNF), ethylanthraquinone (EAQ),and mixtures thereof, 10-20% by weight of polystyrene-oxazolinecopolymer (PS-OX)as a polymer binder, and 20-85% by weight of toluene asa solvent; establishing a substantially uniform electrostatic chargeover the whole area of the inner surface of the photoconductive layer;exposing selected areas of the of the photoconductive layer to dischargethe charge from the selected areas; and developing one of the charged,unexposed areas and the discharged, exposed areas with one of chargedphosphor particles and light-absorptive material particles, dependingupon the polarity of the charged particles.
 6. A method ofelectrophotographically manufacturing a luminescent screen on aninterior surface of a faceplate panel for a CRT, the methodcomprising:coating the interior surface of the faceplate panel with avolatilizable conductive layer and an overlying volatilizablephotoconductive layer, the overlying volatilizable photoconductive layerbeing formed by applying and drying a solution having 2-5% by weight oftetraphenyl ethylene and 0-0.1% by weight of tetracyanoquino-dimethane(TCNQ) as an ultraviolet-sensitive donor material, 0.1-1.0% by weight ofan acceptor selected from the group consisting of trinitrofluorenone(TNF), ethylanthraquinone (EAQ), and mixtures thereof, 10-20% by weightof polystyrene-oxazoline copolymer (PS-OX)as a polymer binder, and20-85% by weight of toluene as a solvent; establishing a substantiallyuniform electrostatic charge over the whole area of the inner surface ofthe photoconductive layer; exposing selected areas of the of thephotoconductive layer to discharge the charge from the selected areas;and developing one of the charged, unexposed areas and the discharged,exposed areas with one of charged phosphor particles andlight-absorptive material particles, depending upon the polarity of thecharged particles.